A reflector antenna aperture illumination function determines the far-field radiative properties of the antenna, specifically the beam-width and sidelobe levels. Together with the spill-over efficiency associated with the reflector feed system and overall geometry, the aperture illumination function also controls the directivity of the antenna. It follows that the antenna design optimization must aim at realizing an aperture illumination function which is consistent with achieving the required far-field performance characteristics. It would thus appear that an adequate selection of antenna geometry and feed design should be able to realize the desired aperture illumination function, however in practice that is often not possible through the use of conventional design techniques.
Multiple-spot-beam antennas are an example of designs for which each feed horn diameter is limited by the constraints of a tightly packaged multi-horn feed assembly. In that case, the limited horn aperture size is often insufficient to enable achieving the desired aperture illumination function, and particularly the illumination edge taper characteristic of a low sidelobe design. Moreover, the optimal aperture illumination function for simultaneous sidelobe and beam-width control is often not a smooth monotonic decreasing intensity towards the edges of the aperture, as can be achieved by excitation with a single feed horn, but rather has variations in both the illumination function and its derivative which are unachievable using realizable feed elements.
Multi-frequency designs are another example for which the desired illumination functions are not easily achieved simultaneously for all frequencies. A typical problem encountered by the designer is that multi-frequency feeds excite the reflector aperture with different illumination functions at different frequencies, resulting in different far-field characteristics at those frequencies, whereas similar performances, including beam-width, directivity, and sidelobe levels, are usually desired. Often the design is geared to favor one of the frequency bands, commonly the lowest frequencies at which the antenna directivity will naturally tend to be lowest, to the detriment of the performance at other frequencies. The ideal situation would be instead to be able to design multi-frequency feeds generating reflector aperture illumination functions which are different at the different frequencies but in a controlled manner, so as to closely compensate for the different aperture diameter-to-wavelength ratios. Multi-frequency feed designs are therefore a critical factor in achieving similar performance at different frequencies, however feed optimization, albeit able to push the reflector antenna design closer to multi-frequency performance equalization, is unable to meet the most ambitious design requirements.
U.S. Pat. Nos. 6,140,978 and 6,421,022 granted to Patenaude et al. on Oct. 31, 2000, and Jul. 16, 2002 respectively, and U.S. Pat. No. 6,563,472 granted to Durham et al. on May 13, 2003 disclose frequency-selective patterns integrated into the design of reflectors. U.S. Pat. No. 6,759,994 granted to Rao et al. on Jul. 6, 2004 discloses a partially reflective surface also integrated into the design of the reflectors. Although modifying the construction of the reflector allows a certain control of the reflector illumination, the solutions proposed by Patenaude et al., Rao et al. and Durham et al. are generally expensive and relatively complex to design, manufacture and test. They are also more susceptible to thermo-elastic distortions since the coefficient of thermal expansion for the outer section of the reflector is typically much higher than that of carbon fiber. This disadvantage makes it unattractive for high frequency applications such as Ka-Band.
Accordingly, there is a need for an improved reflector aperture illumination control device to enhance the overall performance of an antenna.